Contemporary automotive air conditioning systems typically use parallel flow condensers, other heat exchangers, and gas coolers which are used on CO2 systems that are fabricated with extruded tubing. This tubing, which is referred to as micro-multiport (MMP) tubing, is generally made from 1XXX or 3XXX Al alloys. The tubing is a flat body with a row of side-by-side passageways, which are separated by upright webs. Processing of this tubing involves extrusion, a straightening, sizing and cutting operation, assembly and furnace brazing. Brazing is generally done at 600xc2x0-605xc2x0 C. (about 94% of the melting temperature of pure Al). The typical tube straightening and sizing operation imposes a small amount of cold work, in the critical range, which causes extremely coarse grains to grow during the brazing process.
Material handling involves winding the tube on coils and transferring these coils to a straightening and cutting operation. It is during this operation that the final width, thickness and length dimensions of the cut pieces are achieved. The cut pieces are then assembled into a condenser core with fin stock and headers that are clad with a brazing alloy. This assembly is brazed at 600 to 605xc2x0 C.
The production of automotive condensers from aluminum MMP tubing involves an interaction of the tubings and process conditions that can result in undesirable material properties. The combination of a small amount of cold work and the high brazing temperature that must be imposed on the tube cause extremely large grains to form, and this has a significant effect on mechanical properties.
Small amounts of cold work are imposed on the tube during straightening/sizing and material handling. This small amount of deformation can lead to a phenomenon in which very large grains in the aluminum are formed during the brazing process. If a critical amount of cold work is imposed on the tube prior to brazing, then extremely large grains will form after recrystallization. The critical amount of cold work is defined as the amount of strain just necessary to initiate recrystallization. Since few nuclei are formed in the metal, the growth of relatively few recrystallized grains is allowed to proceed with minimum resistance. Conversely, as the amount of cold work increases, more nuclei are produced and the recrystallized grain size decreases.
This invention improves the grain size and the metallurgical strength of the tube by cold working the tubes and controlling the grain size. A multivoid heat exchanger tube is extruded from aluminum alloy billet. Tube dimensions, particularly the size of internal voids are limited by how small extrusion dies and tooling can be manufactured, specifically the mandrel which forms these voids. To achieve ultra small voids in the tube that cannot be achieved with extrusion alone, the tube is put through a rolling process which allows extremely small voids of varying shapes to be formed in the tube. Port shapes that can be formed approximate circles, ellipses, squares and rectangles. The internal walls (sometimes called xe2x80x9cweb wallsxe2x80x9d) can be extruded with a concave shape to achieve the desired shape after extrusion. Rolling thickness reduces the tubes to achieve the desired dimensions above ten (10) percent. The reduction in thickness of the tube and the strain resulting from the cold working imparts the desired strength in the tube.
In FIG. 3, as shown, a multivoid tube prior to cold working has a thickness of a 1.33 mm and port diameter of approximately 0.75 mm.
In FIG. 2, the rolled tube now has a thickness of 0.94 mm and an average port diameter of approximately 0.35 mm.
Accordingly, this invention provides an improved process for enhancing the metallurgical strength of a multivoid tube for use in a heat exchanger. The invention provides a multivoid tube which includes webs between the ports that are configured such that when there is at least a ten percent change in material thickness, the strain from cold working of the tube is concentrated at the center of the webs to improve the strength of the tubing and maintain the desirable small grain growth in the metal tube.
Further objects, features and advantages will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.